Reinforced porous ceramic bone prosthesis

ABSTRACT

A structural member having a core of porous material permitting fluid flow and an outer layer of composite material having rigid matrix with reinforcing material embedded therein. The member finds its primary use in prosthetics.

I United States Patent [151 3,662,405

Bortz et a1. May 16, 1972 54] REINFORCED POROUS CERAMIC 2,679,245 5/1954Timmermans ..128/92 CA 3,314,420 4/1967 Smith et a1. ...l28/92 R BONEPROSTHESIS 3,462,765 8/1969 Swanson ..3/1 [72] Inventors; Seymour A,B0111, Highland Park; Harold 2,210,424 Morrison A Recme" F William RemusFOREIGN PATENTS OR APPLICATIONS Western Springs; Seymour Bazell,Chicago, all of 111. 1,122,634 5/1956 France ..128/92 C [73] AssigneezResearch lnsmute, Chicago, "L 1,500,461 9/1967 France ..3/D1G. 3

[22] Filed: Mar. 12, 1969 Primary Examiner-Richard A. Gaudet AssistantExaminer-Ronald L. Frinks 1 PP 806,578 Attorney-Fitch, Even,Tabin&Luedeka 52 us. Cl ..3/1, 106/40 R, 128/92 c [57] ABSTRACT [51]Int. Cl. ,.A6lf 1/24 A structural member having a core of porousmaterial per- [58] Field of Search ..3/l; 128/92, 92 C, 92 CA, 92 F,mitting fluid flow and an outer layer of composite material 128/92 G;32/10 A; 106/40 R having rigid matrix with reinforcing material embeddedtherein. The member finds its primary use in prosthetics. [56]References Cited 3 Claims, 2 Drawing Figures UNITED STATES PATENTS 24 7448,745 3/1891 Wright ..32/10 A PATENTEDMAY 16 1972 FIG.2

ATTYS.

SEYMOUR BAZELL /M% INVENTORS SEYMOUR A BORTZ 1 REINFORCED POROUS CERAMICBONE PROSTHESIS This invention relates to structural members used as aprosthesis for human bone and made from composite members. Moreparticularly it is directed to structural members made of compositematerials including a matrix material and a reinforcing materialembedded in the matrix.

A composite material is made up of a matrix phase and a discontinuousphase. Normally the matrix phase is a continuous phase and thediscontinuous phase is made up of discrete pieces of reinforcingmaterial. The reinforcing material may be in the form of filaments,fibers, flakes or powder and is added to the continuous matrix phase tolend some desirable property to the composite material not possessed bythe matrix phase alone. For example, if the matrix phase is extremelyresilient the reinforcement might be added to provide additionalstrength while retaining the resiliency of the matrix material.

In the case of ceramic composites the discontinuous phase orreinforcement may be provided for any number of reasons. The generalproperties of ceramic material can best be described by the termsmechanical brittleness, low tensile strength, rigidity, and hightemperature strength. A ceramic has no appreciable yield strength andexhibits brittle failure. It is known that the brittleness and lack ofplastic behavior in ceramics can be modified by the addition of adiscontinuous phase of reinforcing material such as mild steel. When theproportions are correct, the resultant composite exhibits anelastic-plastic stress-strain curve similar to that exhibited by ductilematerials. At the same time it retains many desirable properties ofceramic such as controllable porosity and chemical inertness when anon-reactive reinforcement is used.

As previously mentioned, the present invention finds particularapplication in the growing field of prosthetics. Bone implants arebecoming an increasingly common thing in modern medicine. Several bonesubstitutes are already available for implant. Among the materialsemployed have been stainless steels, ceramics and plastics. Each ofthese materials suffers some limitations when used as a prosthesis. Manyplastics are subject to attack by body chemicals or are rejected by thebodys natural rejecting mechanism. Stainless steel is non-reacting butit is not well adapted to a knitting with the remaining bone structurein the case of splicing since it is not porous. There is furtherdifficulty in adapting the stainless steel to the socket of an existingjoint without having to replace the socket itself. Ceramic materialshave been found to be too weak and brittle for the constant flexuralstress applied to bones. The novel concept presented herein overcomesthese and other difiiculties encountered in the implant of a prosthesis.

A better understanding of the present invention will be facilitated by abrief explanation of the function and structure of human bones, whichare, of course, the principal structural members of the human body. Anatural bone derives its strength from an outer layer of relatively hardmaterial. The material is strong under tensile loading, is durable underflexural loading and is to a certain extent flexible. Inside the outerlayer is a softer area of the bone which does not provide muchstructural support but rather supplies the life needs of the bone. Bloodand other body fluids flow within this inner section ofthe bone.

Bones are attached to each other at joints by means of muscles, tendonsand ligaments. In a typical moving joint such as the elbow or the hip,the ends of two bones are placed together in sliding contact withcartilage and lubrication between them. Under normal circumstances, thejoint will undergo thousands of flexures in a lifetime withoutdeterioration. The durability of the joint is not due solely to thepresence of the cartilage, which is a gelatin-like material. The chiefprevention from wear and tear on the cartilage is provided by naturalbody lubrication which, in a natural joint, is provided by a fluidsecreted by the body and called synovial fluid. The major constituent insynovial fluid is the mucin molecule, a cement substance combined with aprotein to form an elongated molecule. The mucin molecules exist aspolymers of variable lengths. In addition to the mucin molecules, thesynovial fluid also contains glucose, amino acids and other cellularnutrients. Therefore the synovial fluid not only acts as a lubricant butalso nourishes the cartilage. When the circulation of synovial fluid isstopped for a period of time, the cartilage cells in the region nearestthe surface of contact die, eventually causing a bearing type failure ofthe bone at the joint. This is true even though an artificial lubricantis applied if no nutrition is received by the cartilage.

From the foregoing, it is apparent that any prosthesis which is to be ofmore than limited success without the total replacement of the cartilageand the mating half of the joint must in some way permit or even assistin promoting the continued presence of synovial fluid in the joint.

Accordingly, an object of the present invention is to provide animproved composite reinforced bone prosthesis.

A further object of the present invention is to provide an improved boneprosthesis made of ceramic composite material.

Still a further object of the invention is to provide an improved boneprosthesis designed to accommodate predetermined stresses through theengineering of the reinforcement phase of a ceramic composite.

Another object of the present invention is to provide an improved boneprosthesis having high strength chemical inertness and having bearingsurfaces capable of receiving and retaining a fluid.

Other objects and advantages of the invention will be apparent from thefollowing description taken in connection with the accompanying drawingsin which:

FIG. 1 is a partial sectional view taken through a member embodying thefeatures of the present invention; and

FIG. 2 is a sectional view of a prosthesis embodying the presentinvention.

Briefly the present invention relates to a composite article 10 asillustrated in FIG. 1. The article, regardless of its ultimate use orconfiguration, includes bearing surfaces at spaced points on thearticle. For convenience, the article is shown in FIG. 1 as a straightcolumn having the components common to other contemplatedconfigurations. In the illustrated embodiment an inner core 12 of porousmaterial is provided which extends from one end of the column to theother. The core 12 is of sufi'icient porosity to permit fluid to beforced through the pores from one end of the column to the other underpressure. The core 12 is surrounded by a composite ceramic material 14.The composite material is illustrated in the form of a circular cylinderwhich is preferably bonded to the core 12. The composite materialpreferably has a continuous matrix 16 of ceramic material with elongatereinforcing material 18 embedded therein. In the embodiment illustrated,the elongate reinforcing material 18 is oriented in parallel fashionrunning lengthwise of the column.

As briefly described above, the structure shown in FIG. 1 finds itsprimary use as a prosthesis for human bone. The embodiment illustratedin FIG. 2 shows a prosthesis 20 for a human femur. The core 12a (Thesubscript a is employed for elements in FIG. 2 corresponding to elementsin FIG. 1.) of porous material has as its chief function the retentionand circulation of the bodys natural fluids or other added fluidsthrough the structure of the prosthesis to make the fluids available atthe joints for proper movement. The material employed for the core ispreferably truly porous, that is, the pores communicate to form anetwork through the material to permit fluid to flow under pressure. Anadditional function of the core material is to act as a bearing surfacefor a joint. The joint stresses are primarily compressive and areadequately supported by the porous core 12a.

In order to achieve adequate structural integrity as well as porosity,the core 12a is normally formed by conventional casting of an aqueousslurry in a mold with a chemical binder. Casting techniques arepreferable to foaming which tends to produce a porous but friablestructure. The materials are selected for grain size and consistency toprovide the desired degree of porosity and strength. The temperature andpressure of molding is also chosen to provide the optimum properties ofporosity desired for the flow of lubricant. These parameters are wellknown in the art. At the present time ceramics appear to be best suitedfor forming the central core 12a. The reason for their desirability is acombination of ease of forming and chemical inertness both to rejectionby the body and to attack by body fluids. At the same time porousceramics exhibit high compressive strength needed to support the body'sweight in the joints. It is possible that in the future a plastic orother porous material will be discovered which will be equally suitablefrom the standpoint of strength and chemical compatibility with the bodyfluids.

In a structural application such as a prosthesis for a human femur thecore 12a is preferably formed into joints 22, 24 at each end of theprosthesis and is continuous through the length of the prosthesis (FIG.2). For other lower stress applications in which it is not necessary forthe fluid to be communicated to both ends of the prosthesis a porouscore may be employed for the joint which extends only partially into thestructure of the prosthesis thereby retaining the lubricant though notpermitting flow through the prosthesis.

Many ceramics known today are suitable for fonning the porous corematerial of the present invention. Examples of suitable materialsinclude alumina, synthetic aluminum silicates, glass compositions, andvarious pure mineral silicates. These can be cast in a mold using anaqueous vehicle and an inorganic chemical binder such as sodiumsilicate, phosphoric acid, or aluminum phosphate. They can also bepressed as a free flowing powder by die or isostatic pressing. Thesecast and molded or pressed ceramic bodies can be made resistant to bodyfluids by curing at an elevated temperature of at least 300 F.,preferably 700 to 800 F. The resultant structures possess high porosityand good dimensional stability.

As mentioned previously, the porous material is primarily for theretention and flow of lubricating fluids for the joint. The fluid isretained in the pores and is pumped to the cartilage under pressure in amanner similar to squeezing a sponge. The presence of natural bodyfluids is a particularly beneficial feature of the present invention.Because the natural fluids are disposed near the joint where they cansqueeze onto the cartilage, the necessity of replacing the entire jointand cartilage is avoided. The importance of the natural fluids wasmentioned earlier. They serve the dual function of lubrication andnourishment of the cartilage. Just lubrication is not enough. Withoutnourishment the cartilage disintegrates and the joint will fail.

The outer layer of material is preferably formed of a ceramic matrix 16having fibrous or filamentary reinforcing material 18 disposed withinit. The material employed for the matrix 16 may be the same as ordifferent from the material used in the core 12. The same materialslisted above for the core are among those suitable for the matrix. Thematrix is preferably formed by slip casting techniques. It is usuallydesirable that the matrix 16 be less porous than the core material 12.Porosity in any ceramic is easily controlled by proper selection ofgrain size, distribution and viscosity during forming in a manner wellknown in the art.

The reinforcing material 18 is discontinuous and is embedded in the lowporosity matrix. In the illustrated embodiment reinforcement is providedby stainless steel oriented primarily in the direction of highesttensile stress on the member. The length of the reinforcing material mayvary from relatively short fibers 18 (FIG. 1) to elongated filaments 18a(FIG. 2) extending the length of the member to be reinforced. It mayalso be randomly oriented if in a particular application the stresseswill not be great enough to warrant the expense of orienting the fiber.For reasons of weight, reinforcing material 18 should preferably belimited to approximately percent by weight of the composite 14. Thislimitation may require fiber orientation in certain applications wherehigh strength is required. If fiber orientation is desirable, anytechnique is suitable, such as manual, magnetic or electrostaticorientation depending on the material employed. Other high strengthmaterials may be substituted for stainless steel without departing fromthe scope of the invention. Examples of such materials are glass andboron fibers. As in the case of the matrix 16 and core 12 thereinforcing material 18 should be inert to body fluids since porosity inthe matrix cannot be entirely eliminated.

If an elevated temperature process is employed the coeffcient ofexpansion of the reinforcing material 18 is preferably greater than thatof the matrix. For maximum effectiveness the reinforcing material 18 isthen under tension for normal no load conditions.

The matrix 16 has been described as relatively nonporous. It should bepointed out that there are situations where porosity in the matrix isdesirable. One example of the desirability of a porous matrix is theimplant of a partial bone. This type of implant is in the nature of asplice. If the prosthesis is porous, the natural bone will actually knitwith the prosthesis forming a strong bond of bone interwoven in thepores of the prosthesis. In this application a suitable prothesis mightinclude a ceramic of uniform porosity from center to surface havingreinforcement embedded close to the surface.

It has been previously mentioned that all materials employed in aprosthesis must be immune to attack by body fluids. The chief factor inchemical attack by body fluids is the free chloride ion. Therefore asuitable material whether reinforcement, matrix or core should berelatively immune to attack by chloride ions.

The advantages of the present invention over existing prostheses aremultiple. The materials are all free from reaction with and rejection bythe body. The tensile strength of the ceramic is greatly improved by thereinforcement as is the fiexural strength. The ceramic, even in itsporous state, provides adequate compressive strength for joints whileproviding a path for maintaining natural body fluids in communicationwith the joint. Also, the replacement of the cartilage in the joint isavoided.

The following is an example of a principles of the present invention.

EXAMPLE A cylindrical core was first formed from a blend of an aluminaand phosphoric acid. The alumina was composed of 3 parts of a 48 meshand 1 part of a 325 mesh particle gradation. The acid was of an percentconcentration and admixed in the proportions of 1 part acid to 10 partsof the oxide. The mixture was placed in a rubber bag three-fourths inchin diameter and 5 inches long and the opening was sealed. The bag wasplaced in an isostatic press and subjected to 30,000 psi pressure. Thepressed piece was placed in an oven at C. for 4 hours and cured at 400C. for 4 hours. This core was machined after curing to a properprosthetic configuration and was permeable as formed.

A 325 mesh alumina powder was suspended in water by use of a householdtype mixer and stainless steel fibers were added in the amount ofone-fourth the weight of the alumina. Phosphoric acid was added to make10 percent of the solids. The paste was forced about the core section ina mold to form a thickness of one-fourth inch. This was allowed to dryfor 24 hours in the opened mold, then heated 4 hours at 100 C. and 4hours at 400 C. to cure the covering.

While the foregoing description has been directed primarily to use as aprosthesis the present invention finds other uses in any environmentrequiring structural integrity and nonreactivity with the environment.

We claim as our invention:

1. A prosthesis for human bone comprising a composite material of apredetermined configuration,

said composite material comprising a matrix material and flexible,elongated reinforcing material, said reinforcing material and saidmatrix material being adapted to withstand contemplated tensile andbending forces applied to said prosthesis, and

prosthesis embodying the to withstand contemplated tensile and bendingforces applied to said prosthesis, and

a porous ceramic material attached to said composite material at atleast one extremity of said prosthesis, said porous material beingadapted to act as a lubricable joint,

said porous material forming a central core for said prosthesis andextending through the prosthesis to each surface of said prosthesiswhere a joint structure is desired whereby a receptacle is provided forretention and circulation of lubricants to the joint,

all of said material being chemically inert to body fluids.

2. The prosthesis defined in claim 1 in which said reinforcing materialcomprises stainless steel filaments.
 3. A prosthesis for human bonecomprising a composite material of a predetermined configuration, saidcomposite material comprising a ceramic matrix material and elongatedreinforcing material, said reinforcing material and said matrix materialbeing adapted to withstand contemplated tensile and bending forcesapplied to said prosthesis, and a porous ceramic material attached tosaid composite material at at least one extremity of said prosthesis,said porous material being adapted to act as a lubricable joint, saidporous material forming a central core for said prosthesis and extendingthrough the prosthesis to each surface of said prosthesis where a jointstructure is desired whereby a receptacle is provided for retention andcirculation of lubricants to the joint, all of said material beingchemically inert to body fluids.